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SubjectRe: kernel coding style for if ... else which cross #ifdef
On Fri, May 23, 2008 at 10:42:28PM +0200, Willy Tarreau wrote:
> On Fri, May 23, 2008 at 02:11:43PM -0500, Steve French wrote:
> > A question splitting "else" and "if" on distinct lines vs. using an
> > extra line and extra #else came up as I was reviewing a proposed cifs
> > patch. Which is the preferred style?
> >
> > #ifdef CONFIG_SOMETHING
> > if (foo)
> > something ...
> > else
> > #endif
> > if ((mode & S_IWUGO) == 0)
> >
> > or alternatively
> >
> > #ifdef CONFIG_SOMETHING
> > if (foo)
> > something ...
> > else if ((mode & S_IWUGO) == 0)
> > #else
> > if ((mode & S_IWUGO) == 0)
> > #endif
>
> The second one is dangerous because if code evolves, chances are that
> only one of the two identical lines will be updated.
>
> At least the first one is clearly readable.

I would consider the first one much harder to read since you can _very_
easily miss that the "if" is in an "else" clause and completely misread
the code.

> But if you have tons of
> places with the same construct, it's better to create a macro which
> will inhibit the if branch, which gcc will happily optimize away.
> For instance :
>
> #ifdef CONFIG_FOO
> #define FOO_ENABLED 1
> #else
> #define FOO_ENABLED 0
> #endif
>
> if (FOO_ENABLED && foo)
> something
> else if ((mode & S_IWUGO) == 0)
> ...
>...

I'd also say that's the best solution.

> Regards,
> Willy

cu
Adrian

--

"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed



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